It is generally well known that conductors can be fabricated via depositing metallic nanoparticle suspensions (typical particle size 1 . . . 100 nm) on a substrate and thermally sintering the structure. A typical example are the commercially available silver nanoparticle suspensions which can be printed on plastic or paper substrates and sintered at plastic/paper-compatible temperatures (T<200 C) [for example, silver nanoinks by Cabot Inc. and Harima Chemicals Inc]. The low sintering temperature is based on the dramatic depression of the melting point encountered in nanoparticle systems [see e.g. T. Castro et al. Size-dependent melting temperature of individual nanometer-sized metallic clusters, Phys. Rev. B 42, 8548 (1990)].
The suspensions (liquid containing solid particles) are realized by suitably coating the metallic nanoparticles e.g. using a thin polymer shell. Following the deposition on a substrate and the evaporation of the liquid carrier, the coating between the nanoparticles gives way to sintering of the metallic nanoparticles during thermal treatment. As a result, conductor structures, such as printed circuit boards, can be formed using large volume direct-write printing techniques without the traditional patterning techniques (lithography and etching).
However, the realization of small-linewidth, functional structures in conventional printed electronics is difficult. Typical achievable inkjet minimum linewidths are tens of micrometers, and similar or larger for gravure printing. This is a strong restriction for production of devices such as transistors regarding the performance and the device size.
Patent Application FI 20060697, still unpublished when filing the present application, discloses a novel method sintering nanoparticles using electric field. In this method, the electrical voltage set across the deposited nanoparticle system results in sintering without the need for thermal treatment. FIG. 1 (prior-art) shows the results of our electrical sintering experiment on silver nanoparticle system. For each sintering distance (“gap”) between the sintering electrodes, the bias voltage is continuously increased (the system conductivity simultaneously measured using a small-signal AC-method). At a certain voltage, that is proportional to the electrode gap, a dramatic reduction in resistivity is evident. FIG. 2 (prior-art) shows a scanning electron microscope view of the nanoparticle structure before (FIG. 2 (a)) and after (FIG. 2 (b)) the electrical sintering experiment. The structural transformation induced by the electrical sintering is clearly visible by comparing FIGS. 2a and 2b. In our experiments, the conductivities obtained via the electrical sintering method and the conventional thermal sintering method do not significantly differ.
The publication “Controlled insulator-to-metal transformation in printable polymer composites with nanometal clusters”, S. Sivaramakrishnan, et al., Nature Materials 6, 149 (2007); P. Ho, et al. and Patent Application WO 2007/004033 A2 disclose another method for achieving sintered structures by using electric voltage. In the method, electrical pathways are formed through a layer or nanoparticle material between two conducting surfaces.
US 2004/0085797 discloses a method for changing the state of nano- or microparticles by means of electric DC voltage. The voltage is applied by electrodes located on surfaces of a flexible, gel-like layer containing dispersed particles, whereby the particles orient aligned to the electric field or form clusters, the conductivity of the structure being locally increased. The method is not well suitable for producing non-volatile structures and cannot be used for forming conductor wires on surfaces.
WO 2005/104226 discloses a method for fabricating through-contacts in semiconductor chips by applying a very high (>1 kV) voltage burst through a nanoparticle-containing layer. The method cannot be used for forming conductor wires on surfaces.
US 2007/0099345 discloses a method for producing through-contacts through a panel-shaped composite body containing plastic mass filled with conductive particles. A voltage is applied through the mass for fusing or sintering the conductive particles with each other and for converting the plastic mass into conductive carbon bridges. As a result, a conductive through-contact is formed. The method is suitable for contacting semiconductor chips on different sides of the composite body. However, the method can not be used for producing conductors laterally on a surface of a substrate.
In summary, although the many advantages of the known electrical sintering methods, they do not allow for producing very small-linewidth conductor structures on substrates.